1. Field of the Invention
The invention generally relates to power supply filters. More particularly, the present invention relates to power supply filters used in phase-locked loop circuits.
2. Description of the Related Art
An important part of any computer system is the microprocessor. Today's microprocessors are continually improved to increase the speed and lower the voltages at which computer systems operate. As the execution speed of microprocessors has reached and exceeded 200 MHz and continues to increase, the importance of maintaining a high fidelity timing signal becomes increasingly important.
Microprocessors typically use a clock distribution system to supply timing signals to the various microprocessor operating circuits. The clock distribution system buffers timing signals and supplies timing signals having a suitably accurate frequency and duty cycle and minimum phase error. In high speed microprocessors, the accuracy of frequency and duty cycle reproduction becomes increasingly important. One technique for supplying an accurate timing signal involves generation of a clock signal by a phase-locked loop (PLL) and distribution of the clock signal over the large area of the integrated circuit, while maintaining the clock skew to a specified tolerance throughout that area.
A highly accurate timing signal is achieved by ensuring a constant power supply to the PLL and strict limitation of noise to the voltage-controlled oscillator (VCO) within the PLL. Noise in the power supply to the VCO results in jitter that greatly reduces the effective frequency of which the processor can operate. Present day and future microprocessor operating speeds are only attained if the noise in the power supply is highly limited.
Conventional integrated circuits typically use an RC filter to attenuate the power supply noise on an external dedicated VDD line coming into the integrated circuit chip dedicated VDD for the PLL. The RC filter typically includes a variable resistor or potentiometer element for manually adjusting the characteristics of the filter. One problem with conventional integrated circuits and RC filters is that the PLL circuit typically has dual countermanding requirements of a high voltage requirement and a large amount of filtering to the input power supply voltage. A suitable RC filter for the demands of power supply filtering uses a large resistance and typically a large capacitance. The large resistance causes a voltage drop that reduces the power supply level. Thus, only small resistor values are usable which makes the RC filtering ineffective.
Thus, a drawback of current power supply filters is that they fail to provide adequate filtering at lower VDD voltages because the resistance needs to be small. What is needed is a power supply filter that functions at low power supply voltages.